Question

Draw an 18 -carbon saturated fatty acid. Is this a "straightchain" molecule or a "bent" molecule?

Short answer

It's a straight-chain molecule (stearic acid).

Step-by-step solution

Identifying the Molecule

An 18-carbon saturated fatty acid is also known as stearic acid. It has the molecular formula of C₁₈H₃₆O₂.

Structuring the Carbon Chain

In a saturated fatty acid, all carbon atoms are connected by single bonds. Therefore, for stearic acid, draw a straight chain of 18 carbon atoms.

Adding Hydrogen Atoms

To ensure all carbons have a complete octet, attach hydrogen atoms. Each carbon atom should have enough hydrogens to make four bonds in total.

Adding the Functional Group

At one end of the carbon chain, add a carboxylic acid group (-COOH) to complete the fatty acid structure. It replaces one hydrogen at the first carbon.

Determining the Shape

Since stearic acid is a saturated fatty acid with single bonds only, it does not have kinks or bends that are typical in unsaturated fatty acids. Therefore, it is a straight-chain molecule.

Key concepts

Stearic Acid

Stearic acid is a common type of saturated fatty acid consisting of 18 carbon atoms, making it a long-chain fatty acid. It is known for being solid at room temperature, which differentiates it from many other fatty acids that remain liquid. Stearic acid is widely found in various animal and plant fats, such as in cocoa butter and beef fat. It is characterized by having no double bonds between carbon atoms, which contributes to its saturated nature. This property also influences its texture and melting point.

• Chemical Formula: C₁₈H₃₆O₂
• Common Uses: Manufacturing soaps, cosmetics, and candles

Stearic acid's lack of double bonds means it forms a straight chain, leading us to the next important topic in understanding its structure.

Carbon Chain

In stearic acid, the carbon chain is central to its structure and properties. Since it is saturated, all carbon atoms are linked by single bonds. This means that each carbon atom is bonded to four other atoms, completely saturating the carbon chain without any double or triple bonds. This saturation gives it stability and a straight form.
The carbon chain serves as the backbone of the fatty acid molecule. Every carbon is connected in a sequential manner from one end to the other. In the case of stearic acid, we have 18 carbon atoms linked together.

• Sequential alignment of carbon atoms
• Complete saturation with hydrogen atoms ensuring single bonds

It's important to understand this arrangement as it also affects the fatty acid's physical and chemical behavior.

Carboxylic Acid Group

At one end of the stearic acid molecule is the carboxylic acid group. This functional group consists of a carbon atom double-bonded to an oxygen atom and single-bonded to a hydroxyl group (-COOH). It plays a critical role in defining the chemical properties of the fatty acid.
The presence of the carboxylic acid group allows stearic acid to participate in reactions typical for acids, such as forming esters and salts. This group also contributes to the slightly acidic nature of stearic acid in aqueous solutions.

• Essential for the fatty acid's acidic properties
• Site for typical chemical reactions

Understanding the carboxylic acid group helps illustrate how fatty acids engage with other molecules in chemical processes.

Straight-Chain Molecule

Stearic acid qualifies as a straight-chain molecule. This means the chain of carbon atoms in the molecule represents a linear configuration without any bends or rotations. The lack of double bonds in saturated fatty acids like stearic acid gives them this linear quality.
In a straight-chain molecule, the geometric alignment allows for tight packing, leading to higher intermolecular forces. This characteristic is one reason why stearic acid solidifies at room temperature. This straight formation also influences how these molecules stack together, affecting their melting point and physical state.

• Linear arrangement aids in solidification
• Higher stability due to lack of kinks or bends

Thus, the structure of a straight-chain molecule is crucial to understanding the physical properties and behavior of saturated fatty acids like stearic acid.